Surface Marking of the Greater Auricular Nerve
I cannot provide a diagram in this text-based format, but I can describe the precise anatomical landmarks you need to locate the greater auricular nerve (GAN) on the surface of the neck.
Key Surface Landmarks
The greater auricular nerve emerges at a point (the "great auricular point" or GAP) on the posterior border of the sternocleidomastoid muscle (SCM), located approximately 6 cm below the mastoid process. 1, 2
Specific Measurements from Bony Landmarks:
- Distance from mastoid process to GAP: 60.4 mm (±13.76 mm) along the posterior border of the SCM 2
- Distance from angle of mandible to GAP: 57 mm (±22.2 mm) 2
- Distance from tragus to GAP: 6.93 cm (±1.55 cm) 1
Relationship to SCM Length:
The GAP is located at a predictable ratio along the SCM:
- The GAP occurs at 33-57% of the total SCM length when measured from the mastoid insertion 2
- In 66.7% of individuals (Type A), the GAP is above the midpoint of the SCM 2
- In 33.3% of individuals (Type B), the GAP is below the midpoint of the SCM 2
The M-A Line Method (Anterior Triangle):
For locating the nerve anteriorly near surgical fields:
- Draw an imaginary line from the mastoid tip to the angle of the mandible (M-A line) 3
- The GAN crosses this line at its middle third, approximately 48.2% (±6.9%) of the distance from the mastoid tip 3
- The "anterior greater auricular point" (AGA) where the nerve crosses the anterior border of the SCM is located:
Relationship to External Jugular Vein:
- The GAN runs approximately 1.67 cm from the external jugular vein 1
Course After Emergence
After emerging at the GAP, the nerve:
- Courses over the SCM in a superoanterior direction toward the parotid gland and auricle 1, 4
- Bifurcates in 55.6% of cases (most commonly in the anterior third of the SCM) 1
- Trifurcates in 4.4% of cases 1
- Remains undivided in 40% of cases 1
Clinical Relationship to Accessory Nerve
The GAN serves as a reliable landmark for the spinal accessory nerve: